A chelating agent contains two or more electron donor atoms that can form coordinate bonds to a single metal atom. Chelation is an equilibrium system involving a chelating agent (chelant), the metal, and the chelate. The equilibrium constants of chelation are typically orders of magnitude greater than are those involving complexation of metal atoms by molecules having only one donor atom. Chelating agents, broadly, are used to control metal ion concentrations. Chelates (the chelation complexes) have properties that are different than those of both the chelating agent and the chelated metal ion. Chelating agents can thus be used to reduce undesirable properties of metal ions by sequestration. Alternatively, chelating agents can be used to produce desirable effects, for example in waste water treatment, metal buffering, corrosion inhibition, solubilization, treatment for metal intoxication, and cancer therapy.
There are a large number of chelating agents currently known. A number of known chelating agents are listed in the Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, John Wiley and Sons (1991), the chapter entitled "Chelating Agents", Howard, W., and Wilson, D., Vol. 5, at pp. 768-769.
Most of these chelating agents are expensive to produce, usually derived from relatively expensive petroleum-based starting materials, and derived by relatively complex procedures from the starting materials. Accordingly, most chelating agents are not adapted for use in commercial processes requiring an abundant supply of relatively inexpensive chelating agents. Rather, they are suited only for use as analytical reagents.
In addition to the expense of many of the chelating agents, a number of the chelating agents are at most very slightly soluble in water, making them inappropriate, or too expensive, to employ in water-based environments. Additionally, many of the chelating agents are not stable at elevated temperatures of 100.degree. C. or greater, or in high-energy conditions such as in radioactive environments. Additionally, many of the chelating agents are not stable and perform poorly in acidic conditions, and are not oxidation stable. Each of these limitations affects the practical uses of any given chelating agent.
There are a number of situations in which there is a great need for a chelating agent which is inexpensive enough to be used in an industrial process, yet robust enough to withstand elevated temperature/high energy conditions, and/or acidic, and/or oxidizing, conditions. Examples of these types of environments include removal of radioactive materials from low level and high level radioactive liquids. For example, currently much effort is being made to concentrate low level radioactive liquid waste. The metals separation today is typically accomplished by membrane filtration. Spent fuel rods from nuclear power plants are typically cleaned with strong oxidants, such as nitric acid, at pHs of less than one. It would be highly desirable to employ a chelating agent to sequester the radioactive substances from the cleaning solution in order to decrease storage costs. However, most chelating substances are unstable and perform poorly in the highly acidic condition in which this waste exists, and/or are too expensive to be used in such an industrial process.